Lidstock material having improved burst strength

ABSTRACT

A lidstock material suitable for sealing plastic containers that are retorted at elevated temperatures to sterilize their contents. The lidstock material comprises a substrate joined to a film comprising a mixture of a butene-1 polymer; high density polyethylene; polypropylene; and a powdered filler. The lidstock material is heat sealable, peelable, and retains high burst strength both during and after retorting at elevated temperatures.

FIELD OF THE INVENTION

[0001] The present invention relates to a lidstock material suitable formaking lids to be sealed over plastic containers that are heated tosterilize their contents.

BACKGROUND OF THE INVENTION

[0002] Products that must be heated after being packaged in order tosterilize their contents include disposable contact lenses and some foodproducts such as meats. Disposable contact lenses are usually packagedin blister packages consisting of 2 pieces: a base and a lid. The baseis an injection molded plastic shaped to include a bowl-shaped orrectangular recess for receiving the contact lens. Each blister packgenerally contains a contact lens and enough solution to prevent dryingand to maintain the contact lens ready for use.

[0003] The lid covering the base is heat sealed over the recesscontaining the contact lens. Then the package is retorted in anautoclave to sterilize the contents. A lidstock material for the lidmust possess sufficient burst strength during and after autoclaving tokeep the package sealed so that contact lens solution does not escapethroughout the shelf life of the blister pack. The lid must also bepeelable from the base in order to provide easy access to the contactlens held within the blister pack.

[0004] Lidstock materials suitable for covering openings in packages forcontact lenses and foods are known in the prior art. However, the priorart lidstock materials generally suffer from one or more seriousdisadvantages making them less than entirely suitable for their intendedpurpose.

[0005] A principal objective of the present invention is to provide alidstock material for lids requiring improved burst strength.

[0006] A related objective of the invention is to provide a lidstockmaterial for making heat sealable and peelable lids on plasticcontainers.

[0007] Additional objectives and advantages of the present inventionwill become apparent from the following detailed description of somepreferred embodiments.

SUMMARY OF THE INVENTION

[0008] In accordance with the present invention there is provided apeelable and heat sealable lidstock material suitable for making lidsfor plastic containers. Plastic containers sealed by lids made inaccordance with the invention are used for holding disposable contactlenses. Such containers are also suitable for holding meats such asbeef, lamb, pork, and poultry, that are sterilized by retorting afterbeing sealed in their containers.

[0009] As used herein, the term “lidstock material” refers to a metal,polymer, or paper substrate laminated with a heat seal layer or film.Lidstock material of the present invention is made into container lidsby cutting the material into desired shapes such as rectangles. As usedherein, the term “heat sealable” refers to the ability to form a bondbetween a plastic container and its lid when heat and pressure areapplied locally for a sufficient time. The bond is gas-tight andpreferably has sufficient burst strength to resist separation of the lidfrom the container body, even when the sealed container is retorted atan elevated temperature.

[0010] As used herein, the term “peelable” refers to the ability of asealed lid to separate and to release from sealed engagement with acontainer body while both the lid and the body substantially retaintheir integrity. Such separation and release are achieved by manuallyapplying a separating force to an outer edge portion of the lid.

[0011] The lidstock material of the invention preferably a metalsubstrate laminated with a film comprising a polymer mixture. Thesubstrate is preferably an aluminum foil having a thickness of about0.25 mil to 3.0 mils (0.00025 inch to 0.003 inch). Aluminum foil ispreferred because it provides an excellent barrier against penetrationof gases and moisture. Aluminum foil also protects the package contentsfrom ultraviolet light and has an aesthetically pleasing appearance. Aparticularly preferred aluminum foil substrate has a thickness of about2.0 mils (0.002 inch). Other suitable materials for the substrateinclude biaxially oriented polyethylene terephthalate (PET), nylon,paper, and combinations thereof.

[0012] The aluminum foil substrate is preferably coated with a printprimer. The print primer facilitates application of printed labeling onthe substrate. A particularly preferred print primer has a coatingweight of about 0.7 pound per 3000 square feet.

[0013] The peelable and heat sealable film has a total weight of about15-30 pounds per 3000 square feet. A particularly preferred film has aweight of about 17 pounds per 3000 square feet of the lidstock material.

[0014] The container body preferably comprises an injection moldedpropylene homopolymer. Other suitable plastics for the container bodyinclude polyethylene; polyethylene-polypropylene mixtures; andpolyethylene-polypropylene copolymers.

[0015] The film in the lidstock material comprises a mixture of abutene-1 polymer; polyethylene; polypropylene; and a particulateinorganic filler.

[0016] Some suitable inorganic fillers include talc, amorphous silicaand alumina trihydrate. The filler enhances peelablity of the coating byshifting seal failure upon peeling from adhesive failure at thecontainer-coating layer interface to cohesive failure in the coatinglayer itself. The filler comprises at least about 18 wt. % of thecoating, preferably about 20-40 wt. %, more preferably about 20-30 wt. %and optimally about 25 wt. %. The filler is preferably a powder havingan average particle size of about 0.5-10 microns. Talc having an averageparticle size of about 1-2 microns is particularly preferred. The talcshould be provided with a surface coating comprising about 0.5-5 wt. %of the filler, preferably about 1 wt. %. A carboxylic acid surfacecoating is particularly preferred.

[0017] The carboxylic acid in the surface coating may be a mono ordicarboxylic acid or a mixture of such acids. Some preferred acidsinclude stearic acid and isostearic acid, which is a liquid mixture ofmostly C₁₈ saturated fatty acids having the general formula C₁₇H₃₅COOH.Other saturated C₁₀-C₂₀ carboxylic acids or mixtures thereof may also beuseful.

[0018] The mixture preferably comprises about 15-25 wt. % of a butene-1polymer, more preferably about 20-35 wt. % of a butene-1 homopolymer. Aparticularly preferred polybutylene sold by Basell Polyolefins as “PB0110” polybutylene has a density of 0.915 g/cm³; a melt flow rate of0.400 g/10 min by ASTM D 1238; and a melting point of 257° F. (125° C.).

[0019] The mixture preferably comprises about 35-55 wt. % high densitypolyethylene (HDPE), more preferably about 35-55 wt. %. The HDPE ispreferably M6210 HDPE, available from Lyondell Petrochemical Company.

[0020] The mixture also comprises about 5-15 wt. % polypropylene.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021]FIG. 1 is a top plan view of a blister pack made in accordancewith the present invention.

[0022]FIG. 2 is a side elevational view of the blister pack of FIG. 1with the lid partially peeled back.

[0023]FIG. 3 is an enlarged fragmentary, cross-sectional view of the lidof the blister pack shown in FIG. 2.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0024] In FIGS. 1 and 2 there is shown a blister pack 10 made inaccordance with the present invention, with a lid 12 partially peeledback to reveal its contents. The pack 10 comprises an injection moldedplastic body 14 heat sealed to the lid 12. The body 14 defines abowl-shaped recess 16 having a diameter of about 2 cm. and a depth ofabout 0.5 cm. A flange 18 extending around the recess 16 includes atapered curled lip 20 spaced apart from the recess 16.

[0025] The recess 16 houses a contact lens 22 and a saline solution 24.The recess 16 is circumscribed by a seal area 26 which is part of theflange 18. The lid 12 is preferably attached to the body 14 by heatsealing in the seal area 26. Other suitable means for attaching the lid12 to the body 14 include induction sealing and sonic welding. The totalinterior volume defined by the recess 16 and the lid 12 is preferablyless than 1 milliliter.

[0026] The body 14 is preferably made from a plastic material which canbe shaped by injection molding or thermoforming. The plastic materialfor the body is preferably polypropylene but may also be other plasticmaterials having similar properties, such as polyethylene, polybutylene,polyesters (e.g. PET), polycarbonates, and other thermoplastics.Plastics having low vapor transmission rates are most preferred.

[0027] Referring now to FIG. 3, the lid 12 is made from lidstockmaterial comprising an aluminum foil substrate 30 having a polyurethaneadhesive layer 31 joining a film 32 to the substrate 30. An exteriorside of the substrate 30 displays graphic matter (not shown) printedover a print primer 33. The foil 30 has a thickness of about 2 mils.(0.002 inch). The print primer 33 has a weight of about 0.7 pounds per3000 square feet. The film 32 has a thickness of about 1 mil.,corresponding to a weight of about 17 pounds per 3000 square feet. Thelid 12 includes only a single layer of the film 32.

[0028] The preferred blister pack 10 shown in FIGS. 1-3 has a lid 12heat sealed to an injection molded polypropylene body 14. The film 32 onthe lid 12 may have either of the particularly preferred compositionsshown in the following table. Film Composition Amount (wt. %) IngredientA B Butene-1 homopolymer 25 15 High density polyethylene 35 55Polypropylene 15  5 Talc 25 25 Total 100  100 

[0029] Tests performed on blister packs made in accordance with theforegoing examples demonstrate that the lidstock material of theinvention has excellent burst strength, both during the sterilizationprocess and after the packages are sterilized by exposure to severalcycles at 120° C. Among other requirements satisfied by the lidstockmaterial are impermeability to bacteria so that sterility of thecontents is preserved, a negligible vapor transmission rate to avoidloss of water, ability to maintain the lens in its original conditionfor an expected shelf life of about 2-4 years, and low variation in peelstrength over an expected range of heat seal temperatures.

[0030] Having described the presently preferred embodiments, it is to beunderstood that the invention may be otherwise embodied within the scopeof the appended claims.

What is claimed is:
 1. A lidstock material comprising a solid substratelaminated with a film comprising: a.) about 15-25 wt. % of a butene-1polymer; b.) about 35-55 wt. % high density polyethylene; c.) about 5-15wt. % polypropylene; and d.) at least about 18 wt. % of a particulateinorganic filler.
 2. The lidstock material of claim 1, wherein, saidsubstrate comprises at least one material selected from metal foil,plastic, and paper.
 3. The lidstock material of claim 1, wherein saidlidstock material comprises aluminum foil.
 4. The lidstock material ofclaim 1, wherein said inorganic filler comprises talc, silica, oralumina.
 5. The lidstock material of claim 1, wherein said filmcomprises about 20-30 wt. % of the inorganic filler.
 6. The lidstockmaterial of claim 5, wherein said filler comprises about 0.5-5 wt. %(based on the weight of the filler) of a carboxylic acid.
 7. Thelidstock material of claim 1, wherein said lidstock material is peelableand heat sealable.
 8. The lidstock material of claim 1, wherein saidlidstock material has improved burst strength both during and afterretorting at elevated temperatures.
 9. The lidstock material of claim 1,wherein said film comprises about 35-50 wt. % high density polyethyleneand about 20-30 wt. % inorganic filler.
 10. The lidstock material ofclaim 9, wherein said filler comprises talc having an average particlesize of about 0.5-10 microns.
 11. A lid for a plastic containercomprising the lidstock material of claim
 1. 12. A container comprisinga plastic body having an edge defining an opening and a flange extendingradially outwardly of said opening, and a lid comprising the lidstockmaterial of claim 1 heat sealed to said flange.
 13. The container ofclaim 12, wherein said plastic body comprises polypropylene.
 14. Thecontainer of claim 12, wherein said substrate comprises aluminum foil.15. A film suitable for lamination with a solid substrate to form alidstock material, said film having a weight of about 15-30 pounds per3000 square feet, said film comprising about 15-25 wt. % of a butene-1polymer, about 35-55 wt. % high density polyethylene, about 5-15 wt. %polypropylene, and about 20-30 wt. % of a powdered inorganic fillerselected from talc, silica, and alumina.